Process for production of 1,12-dodecanedioic acid

ABSTRACT

An improved process for production of 1,12-dodecanedioic acid which comprises reductive coupling of the peroxide from reaction of hydrogen peroxide on cyclohexanone by treatment with an aqueous solution of a chelated ferrous compound having a pH in the range of about 1 to 6 and at a temperature in the range of about 20 to 100*C., the iron compounds being maintained in solution throughout the process.

United States Patent Welton Sept. 23, 1975 PROCESS FOR PRODUCTION OF [56] References Cited IJZ'DODECANEDIOIC ACID FOREIGN PATENTS OR APPLICATIONS Inventor: Donald W ilm g 697,506 9/1953 United Kingdom 260/537 P 73 Assignee: E. I. Du Pont de Nemours and Primary Hammer-Vivian Garner Company, Wilmington, Del.

22 Filed: Oct. 23, 1973 [571 ABSTRACT An improved process for production of 1,12- 21 A 1.N..40, 9 1 pp 0 827 dodecanedIoIc acid which comprises reductive cou- Relat d US- Application ata pling of the peroxide from reaction of hydrogen per- [63] Continuation-in-part of Ser. No, 268,769, July 3, oxide on cyclohexanone by treatment with an aqueous 1972, abandoned. solution of a chelated ferrous compound having a pH in the range of about i to 6 and at a temperature in [52] U.S. Cl. 260/537 P the range of about 20 to 100C, the iron compounds [51] Int. Cl. C07C 51/00 being maintained in solution throughout the process. [58] Field of Search 260/537 P 5 Claims, 4 Drawing Figures US Patent K or KA PHK REACTOR COUPLING REACTOR K or M REDUCTANT SOLUTION PHK REACTOR COUPLING REACTOR REDUCTANT SOLUTI O l XTA L Sept. 23,1975 Sheet 1 of 2 3,907,883

HYDROCARBON SOLVENT SOLVENT I SOLVENT COOL ER STILL BECAME msmuzaa REDUCTART 5m" RECEN FILTER K or M STILL CRUDE DOA xm K or M srm JDECANTYER coouan iCRYSTALLIZER STILL REDUCTANT LIQUOR REGEN. mm

at CRUDE DOA US Patent Sept. 23,1975 Sheet 2 of2 3,907,883

FIG-]I[ K or M ALCOHOLIC SOLVENT PHK ALCOHOL l REACTOR SOLVENT A0 H202 K OI KA COUPLING OL STEAM COOLER REACTOR CRYSTALLIZER STILL STILL REDUCTANT SOLUTION FILTER L REDUCTANT LIQUOR REGENERATOR CRUDE mm t XTAL 2 FIG. ]I

AlR-OX|DIZED "A" FEED "A" OXIDATION PRODUCT KA STEAM COOLER COUPLING CRYSTALLIZER REACTOR BECAME STILL REDUCTANT REDUCTANT uquoR SOLUTION REGENERATOR m;

CRUDE DDA XTAL PROCESS FOR r oDUCTlQNor l,lZ-DODE CANEDIQICA CID CROSS REFERENC'ETO RELA E -APPLICATION This is a continuation -in-partlof application Ser. No. 268,769filed July 3, i972, now abandoned by Donald E. Welton. v V v G BACKGROUND OF THE'INVENTION It has been known for some time (U.S. Pat. No. 2,601,223) that l,l2-dodecanedioic acid (DDA) can be produced by a reductive coupling treatment of an organic peroxide such as that obtained by reaction of hydrogen peroxide on cyclohexanone with an appropriate reducing agent such as a ferrous iron compound. A shortcomingof such a preparative method is the tendency of the oxidized form of the reducing agent to be precipitated from the reaction medium during the coupling reaction or in work-up of the product, thus complicating the isolation and purification of the desired DDA as well as making more difficult the reclaiming and recycling of the reducing agent for subsequent operations. The use of added mineral acids to prevent undesired precipitation of the iron reducing agent is described in British Pat. No. 740,747. Accordingly, an improved process free of the above-mentioneddifficulties has been sought.

SUMMARY OF THE INVENTION product (PHK) in a suitable organic solvent is vigorously mixed with an aqueous solution of chelated ferrous iron compound at a temperature in the range of about to 100C., preferably about to 80C., which results in reductive coupling of PHK to form the desired DDA along with oxidation of the chelated ferrous iron to the ferric state as illustrated by the following equations:

CO2H 'co n DDAT An organic solvent phase containing the desired along with by-product caproic acid is separated from the aqueous phase containing the oxidized orspent reductant solution. The DDA is recovered by appropriate procedures. The spent reductant contained in the aqueous phase can be separated and discharged or it can be regenerated by catalytic hydrogenation and recycled to the process.

p The PHK required for the reductive coupling can be ,obtained from any convenient source suchas by reaction of commercial hydrogen peroxide solution with (l) purified cyclohexanone (K), (2) a cyclohexanonecyclohexanol mixture (KA) obtainable from the adipic acid manufacturing process, (3) a cyclohexanecyclohexanone-cyclohexanol mixture obtainable from the adipic acid manufacturing process or it can be obtained in situ by the air oxidation of cyclohexanol.

An indicated above, the reductive coupling reaction is carried out in a similar way with the PHK obtained from thedifferent sources. However, depending on the bulk solventthat is used, variations in the product separations and recoveriesare employed. It is found that bulk solvents such as cyclohexanone or cyclohexanol,

aliphatic, cycloaliphatic or aromatic hydrocarbons such as n-hexane, cyclohexane or benzene and lower alcohols such as methanol, ethanol or isopropanol can be used with the various sources of PHK. Suitable separation techniques for a number of combinations are shown in FIGS. I-IV. The product separations and recoveries are based on the fact that (a) DDA is very insoluble in water, in aqueous reductant solutions or in cyclohexane but is very soluble in alcohols, warm cyclohexanone or cyclohexanol, (b) cyclohexanone and cyclohexanol are only slightly soluble in water but are s team-distillable, (c) the iron chelates employed in the process are soluble in water. 7

With respect to the several separation procedures shown in the Figures, that illustrated in FIG. I employs as a source of PHK either cyclohexanone or a mixture of cyclohexanone and cyclohexanol in cyclohexane along with a commercial 590% aqueous hydrogen peroxide with cyclohexane as the main solvent. in this case all organic products remain in the warm upper layer so that decantation permits separation and recycle of the spent reductant solution. The crude DDA product can be crystallized from the hydrocarbon layer followed by hydrocarbon and cyclohexanone recovery stills.

The system shown in FIG. ll employs as the source of PHK either cyclohexanone and hydrogen peroxide, a mixture of cyclohexanone, cyclohexanol and hydrogen peroxide or the product of air oxidation of cyclohexanol. The bulk solvent is either cyclohexanone, cyclo- 0 OH Ho a rnixture-ofthese. The DDA product re- 3 mixture of cyclohexanoneand cycl'ohexanol'and'crystallizing the desired product from the aqueous tails from the still. I

In FIG. III there is used an alcohol solvent such as methanol, ethanol or isopropa'nol 'with 'the same sources as used in FIG. I or FIG. II. This system reagent characterized by its ability to prevent the precipitation of either ferrous or ferric iron at temperatures from C. to 100C. with iron concentrations fr'or'n'0.1

molar to 1.0 molar at a pH in the range of about l to about 6.

' By chelating agent'is meant a compound containing at least one carboxylic acid group and at least one basic nitrogen containing group having an aminoid nitro'gen or hetroaromatic nitrogen as in pyridine or quinoline derivatives and wherein the carboxylic acid and nitrogen containing group are separated by at least one carbon atom and by not more than two carbon atoms. Operable chelating agents can be defined as being of the group consisting of picolinic acid and agents of the formula XYN(ZCO H) wherein. 2 is .CH CH CH or CHR- wherein R is an alkyl or aryl radical having up to 8 icarbon atoms, wherein X is a radical of the group consisting of. ,H, ZCO H, CH CH N(CH CO H) or l l i and Y is CH CO H. The preferred chelating agents have more than one carboxylic acid-group; apreferred agent is nitrilotriacetic'acid; a preferred chelate is the ammonium salt of the anion ferrous nitrilotriacetate. Other suitable chelating agents include iminodiacetic acid, ethylenediamine tetra-acetic acid, a-substituted nitrilotriacetic acids, nitrilotripropionic acid, N-(2- picolyl)iminodiacetic acid and picolinic acid.

Concentrations of the reductant solution should be as high as solubility and a suitable viscosity will allow. Good working ranges appear to be from 0.25-1.00 molar. More concentrated solutions tend to become viscous, and more dilute ones require excessively large liquid-handling equipment. Solutions'are generally prepared directly from ferrous sulfate heptahydrate, chelating agent and aqueous ammonia, so that sulfate/iron ratio in solution is ordinarily 1:]; there appears to be no need to remove the ammonium sulfate nor is additional ammonium sulfate harmful. Ammonia as the base is generally preferred from the standpoints of high "solubility of both ammonium-iron-chelate and the by-" product ammonium sulfate; potassium hydroxide is' suitable and bases of other alkali metals can be'u'sed as pounds. The most suitable base will depend on the particular chelat'e employed in the process; those producing optimum solubility characteristicswill be preferred.

Chelating agent/iron mol ratio should be the minimum needed to avoid precipitation o f iron salts during recycle; about l:l "seems optimum. Excess chelating agent appears to be attacked by peroxide.

The pH of thesolution should be. in the range of about 1-6 with optimum at 4-5. With solutions of lowerpI-l, yields in the coupling reaction decrease, and

I at higher pI-I considerable dodecanedioic acid is converted to its salt's, complicating recovery and recycle of reductant solution. 7 2L PHK Solution The peroxide source for production of PHK can be (a) aqueous Hi0 of 5-90% strength, preferably 30-70%, (low concentrations introduce excessive water into the system, may be explosive) or (b) the peroxide contained in the product of air oxidation of cyclohexanol.

The cyclohexanone source can be pure cyclohexanone, a mixture of cyclohexanol-cyclohexanone, such as is obtained from air oxidation of c'yclohexane or the air oxidation product of c yclohexanol.

The PHK can be formed in solution from the above reagent sources, or it may be preformed and isolated in crystal form from H 0 and cyclohexanone with or without removal of the water introduced with the H 0 In any case, his preferred to allow the formation of PHK from cyclohexanone and H 0 to proceed to substantial equilibrium values to maximize combined H O, before use in the coupling reaction. Equilibrium can be .achieved by aging 10-60 mins. after mixing,.or accelerated by the addition of a trace of mineral acid such as sulfuric acid. Alternately, excellent coupling yields can be obtained by simultaneously forming and reacting the PHK solution in the coupling reactor by: adding crystalline PHK to a mixture of solvent and reductant solution. i

- The peroxide concentration in the PHK solution may be from about 0.2 mol/I000 g to about 4.0 mol/I000 g. The preferred range is about 0.4 to about 2.0 mols/lOOO g (these correspond to PHK concentrations of about I050%).

The cyclohexanone/peroxide mol ratio can range from 250 (these correspond respectively to PHK crystal and about 5% PHK dissolved in cyclohexanone). The preferred range is 3-5 except when cyclohexanone is the solvent.

The bulk solvent can be an aliphatic, alicyclic or arornatic hydrocarbon, methanol, ethanol or propyl alcohols, c'yclohexanol or cyclohexanone or a mixture of these in any proportion, and can contain water at any level from substantially none up to saturation. Temperature can be from ambient up to approximately C., preferably 3060C.

3. Coupling Reactor The mole ratio of ferrous iron/P in the reductive coupling reaction is in the rangel-IO, preferably 2-4. Ratios less than 1 .are insufficient for complete conversion of peroxide; ratios greater than about 5 merely increase processing costs.

The temperature range in which the reductive coupling is carried out can be from 20-IOOC., preferably about 4O 8OC. The reaction can be carried out under atmospheric or su pe ratmospheric'(autogenous) preswell as quaternary ammonium bases such as the tetra-' methylammonium and "tetraethylarnmonium' comsurefThe reactants should be thoroughly mixed.

4. Recovery of Spent Aqueous Reductant As indicated in FIGS. l-4, the spent aqueous reductant can be separated by decantation or filtration from organic materials. It is only necessary to recover a homogeneous aqueous phase, which may contain up to saturation limits of any of the organic products of'the coupling reactor.

The spent reductant solution comprising mainly fer- I ric chelates can be hydrogenated back to a solution of the ferrous chelates over a suitable catalyst such as commercial palladium on alumina or carbon at a hydrogenation pressure of 0-500 psig, preferably 0-l00 psig, and at a temperature in the range of 25l00C. Other operable hydrogenation catalysts include conventional precious metal catalysts such as platinum, rhodium or ruthenium, either alone or on such supports as alumina, silica, carbon, asbestos and the like, in either a slurry or fixed bed system.

DESCRIPTION OF PREFERRED EMBODIMENTS The preferred embodiments of this invention are further illustrated in the examples and figures to follow. The following abbreviations are used:

Pl-IK peroxyhemiketal of cyclohexanone DDA l,l2-dodecanedioic acid K cyclohexanone A cyclohexanol KA mixed cyclohexanone/cyclohexanol NTA nitrilotriacetic acid P total peroxide, determined by iodide titration XTAL crystalline EXAMPLE 1 CHELATING AGENT SOLUBILITY TESTS To determine the suitability of chelating agents for use with iron salts for reductive couplin'g, tests of individual chelating agents were carried out as described below.

A solution of 7 g. (25 m mol) of FeSO .7I-I O in 25-50 ml H O was prepared in a nitrogen-blanketed flask with magnetic stirring. An appropriate amount (10, 20 or 30 m mol) of test compound was added and the pH was adjusted to 2-6 with either sulfuric acid or ammonium hydroxide. If a homogeneous solution resulted with or without warming to 60C., solubility of the ferrous salt was judged satisfactory; if a significant precipitate was present it was judged unsuitable.

If the ferrous form was suitable, 1 ml (10 111 mol) of 30% H 0 was added. If no significant precipitate occurred, the oxidized solution was heated to boiling. If aprecipitate appeared at any time, the agent was considered suitable.

The agents passing the solubility tests described above when then further tested for their effectiveness in coupling PHK to DDA as described in Example 2.

EXAMPLE 2 CHELATING AGENT COUPLING TESTS The reductant solutions judged to be satisfactory from the solubility tests of Example 1 were tested for their ability to effect reductive coupling of PI-IK to DDA from a simulated cyclohexanol air oxidation product as described below. The results are summarized in Table I.

a. Preparation of Crystalline PI-IK An assayed aqueous solution of approximately 30% H O was mixed with enough cyclohexanone (K) to produce a 2.05-2.10 mol ratio of K/H O The flask was rotated at ambient pressure on a rotary evaporator until the charge had become crystalline. Vacuum (1 Torr) .was then applied and the flask was rotated in a 25C. water bath until the weight dropped to theoretical for PHK (2 K/l peroxide). The product was then pulverized and assayed for peroxide content; theoretical =4.35 m mol peroxide/g. Found (several preparations): 4.2-4.6 m mol P/g.

b. Preparation of PI-IK Solution PHK was mixed with approximately one-half its weight of K and enough A was added to make the PHK concentration 15% by weight. This gave solutions containing 0.6-0.7 m mol P/gram and total K/peroxide ratios about 3/1. The solutions were stirred with slight warming until homogeneous, then let stand at least 30 mins. before use.

c. Preparation of Reductant Solution This was prepared immediately before use. It was prepared in a nitrogen-blanketed stirred reactor using 7.0 g (25 m mol) FeSO .7I-I O and 20-50 ml water plus the appropriate. amount of chelating agent (from screening test) and sulfuric acid or alkali metal or ammonium base is needed to adjust pH to the desired value in the I-6 range. It was kept under N until the coupling reaction was completed.

d. Coupling Reaction Test Enough of the pHK solution to contain 20 m mol of peroxide was added rapidly, under N to the vigorously-stirred reductant solution. The mixture was stirred 5 minutes, then transferred to a separatory funnel and the lower aqueous layer was removed and washed with 5 ml cyclohexanol. The main organic layer and cyclohexanol wash were combined, mixed with approximately 150 ml water and boiled until a vapor temperature of C. indicated substantial removal of all AK. The residual liquid was cooled to room temperature and the crystalline product filtered out, washed repeatedly with water anddried to constant weight. Crude yield, calculated as 100% DDA, from peroxide was determined and the melting point was observed. Theoretical yield of DDA was 2.30 g and m.p. of pure DDA is l29-l 30C.

EXAMPLE 3 EFFECT OF VARIOUS SOLVENT SYSTEMS The results of these tests are summarized in Table II. Nitrilotriacetic acid was the chelating agent chosen for the reductant solution; the PI-IK solutions tested showed the coupling yields attainable from various combinations of feed sources and solvents. In all cases except the last one, the PHK solution was made by reacting cyclohexanone with aqueous 30% H 0 then diluting with solvent. If any aqueous phase separated it was removed before use. In the last case, dry PHK was dissolved in methanol. Reductant was a solution of ammonium Fe -nitrilotriacetate in water, approximately 20% concentration. The Fe /peroxide ratio in the coupling reaction was approximately 1.5/1. The products were assayed for DDA, recovered K and byproducts to determine chemical yields.

EXAMPLE 4 Batch Reductant Recycle Tests These batch tests employed an aqueous solution of ammonium ferrous nitrilotriacetate, 0.43 m mol Fe/g from FeSO .7H O, ammonia and nitrilotriacetic acid. Initial mol ratio of NTA/Fe was approximately 1.05.

The initial test employed 100 g of reductant solution reacted with a PHK solution from 2.0 of 30.5% H (17.7 m mol), 4.0 g. K and ml cyclohexane (decanted from the water layer), both reactant solutions at initial 40C. After reaction the organic layer was separated and assayed. The spent reductant solution was mixed with l g of 1% palladium on alumina catalyst and hydrogenated at 50C./50 psig H pressurefThe regenerated reductant solution was filtered from the catalyst and charged to the coupling reactor for another test. The hydrogenation catalyst was saved for use in regeneration after the next cycle. These operations were repeated seven times, using recycle reductant, without makeup, and the same batch of hydrogenation catalyst, but a fresh batch of PHK solution for each test.

The data are summarized in Table 111. These was no significant change in effectiveness of either the reductant solution or catalyst throughout the test series.

EXAMPLE 5 CONTINUOUS COUPLING AND REDUCTANT RECYCLE TESTS driven stirrer, jacketed with tempered water at 50C., fitted with inlet tubes for the reactants and an outlet tube fitted with a throttling valve to maintain the reactor contents ata constant value of 150 ml. The reductant solution w as fed at approximately ml/min. and the PHK solution at approximately 7 ml/min., Felperoxide mol ratio about 6 and holdup time about 2 minutes. V v

Effluent from the coupling reactor was decanted continuously. The upper organic layer was collected, weighed and assayed for yield data.

The lower spent reductant layer was pumped continuously to a steel regenerator tube containing 300 ml of a commercial 0.2% Pd on charcoal catalyst. 1t operated at 150 psig H pressure at flow rates needed to give approximately 100 STP ml H excess (vent)/minute. Flow of both liquid and gas was upward, with approximately 5 minutes residence time for the liquid. Regenerated reductant exiting the regenerator was returned to the reductant reservoir feeding the coupling reactor. Regenerator temperature was held at C.

This run was operated at steady state for approximately 5 hours. The yield of DDA from peroxide was 48.6% and the'yield of cyclohexanone to DDA was 7 I .3%, to caproic acid 29%. There was no measurable loss of activity of either reductant solution or hydrogenation catalyst.

and continuously decanted from the resultant small water phase. Approximately 95% of the H 0 fed remained in the cyclohexane solution, which was maintained at approximately 0.7-0.8 m mol peroxide/g with 58 wt cyclohexane and a K/peroxide ratio of about 2.53.0. The PHK solution was continuously fed to the coupling reactor. I I

A stock of reductant solution was prepared from Fe- SO .7H O, nitrilotriacetic acid, ammonium hydroxide and water so that the solution contained 0.475 m mol Fe/g and an NTA/Fe ratio of about 1.05 at a pH of.4.5.

TABLE 1 CHELATING AGENT COUPLING REACTION TESTS CRUDE DDA CHELATING AGENT pH Yield from P m.p.."C COMMENTS Nitrilotrincetic Acid (s 53 121) lminodiacetic Acid 4| Ethylencdiamine tetruacetic Acid 26 53 118 a-Benzyl nitrilotriacetic Acid 2.6 48 Agent partly oxidized zri-Butyl nitrilotriacetic Acid 5 44 l 17 Nitrilopropionic-diztcetic Acid 3 46 Picolinic Acid 15 42 I23 N-2-Picolyl lminodincetic Acid 5 24 123 Low solubility ferrous TABLE [I PHK SOLVENT COUPLING TESTS YlELDS PEROXIDF. TOTAL DDA DDA CAPROlC UN- KNOWN CONC. K/P ex ex ex ex SOLVENT m mole/ml RATIO PEROX. K K K Cyclohexanol (wet) L2 2.5 54 27 4 L'yclohcxane 2A/K 0.7 2.3 (1 7X 15 6 lsuprupanol 0.3 2.5 4) 74 1) 7 Cyclohcxane-50'7l K 0.6 7.5 (v2 67 26 -61 Methanol 0.8 5.l 58 71 25 4 Methanol -l dry PHK 0.3 2.0 73 l) 8 TABLE III REDUCTANT RECYCLE COUPLING TESTS 60 w DDA DDA TlME TO ex CAPRO1C AQUEOUS H CYCLE PEROXIDE RATlO -pH UPTAKE 1 59 '2L 7' 4.5 2 59 3.3 4.4 12 65 3 66 4.0 4.3 21 4 69 5.6 4.1 2| 5 58 :'3.7 j 3.9 V I l5 0 65 3.8. 4.0 18 7 51 2.4 4.0 18 Average 61 3.6

This was fed continuously to the coupling reactor.

The coupling reactor was a glass vessel with motor- I claim:

1. In a process for production of 1,12-dodecanedioic acid by reductive coupling of the adduct of eyclohexanone and hydrogen peroxide with an iron salt reducing agent and separating the desired acid from the oxidized reducing agent and by-products of the reductive coupling reaction,

the improvement which comprises effecting the reductive coupling reaction at a temperature in the range of about to 100C. without precipitation of an iron compound at any stage by using an aqueous solution containing a ferrous iron compound combined with a chelating agent, the chelating agent being of the group consisting of picolinic acid and agents of the formula XYN(ZCO H), wherein Z is a radical of the group consisting of CH CH CH and CHR wherein R is an alkyl or hydrocarbon aryl radical having up to 8 carbon atoms, wherein X is a radical of the group consisting of H, ZCO H, CH CH N(CH CO H) and Y is CH CO H, the aqueous solution being maintained at a pH in the range of about I to about 6.

2. The process of claim 1 wherein the chelating agent is of the group consisting of nitrilotriacetic acid, iminodiacetic acid. nitrilotripropionic ethylenediamine tetraacetic acid. a-benzylnitrilotriacctic acid, a-isobutyl nitrilotriacetic acid, nitrilopropionicdiacetic acid, picolinic acid, and N-2-picolyl iminodiacetic acid.

3. The process of claim 2 wherein the reductive coupling is carried out at a temperature in the range of about 25 to C.

4. The process of claim 3 wherein the chelating agent is nitrilotriacetic acid.

5. The process of claim 4 wherein the oxidized coupling agent is regenerated by subsequent catalytic hydrogenation and is thereafter recycled into subsequent reductive coupling reactions.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 1 3,907,883 DATED September 23, 1975 |NVENTOR(S) Donald E. Welton It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 10, line 5, after "nitrilopropionic" add --acid--.

Signed and Scaled this ninth Day Of March 1976 RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner oj'Parents and Trademarks 

1. IN A PRODUCTION OF 1,12-DODECANEDIOIC ACID BY REDUCTIVE COUPLING OF THE ADDUCT OF CYCLOHEXANONE AND HYDROGEN PEROXIDE WITH AN IRON SALT REDUCING AGENT AND SEPARATING THE DESIRED ACID FROM THE OXIDIZED REDUCING AGENT AND BY-PRODUCTS OF THE REDUCTIVE COUPLING REACTION, THE IMPROVEMENT WHICH COMPRISES EFFECTING THE REDUCTIVE COUPLING REACTION AT A TEMPERATURE IN THE RANGE OF ABOUT 20* TO 100*C. WITHOUT PRECIPITATION OF AN IRON COMPOUND AT ANY STAGE BY USING AN AQUEOUS SOLUTION CONTAINING A FERROUS IRON COMPOUND COMBINED WITH A CHELATING AGENT, THE CHELATING AGENT BEING OF THE GROUP CONSISTING OF PICOLINIC ACID AND AGENTS OF THE FORMULA XYN(ZCO2H), WHEREIN Z IS A RADICAL OF THE GROUP CONSISTING OF-CH2-, -CH2CH2- AND -CHR WHEREIN R IS AN ALKY OR HYDROCARBON ARYL RADICAL HAVING UP TO 8 CARBON ATOMS, WHEREIN X IS A RADICAL OF THE GROUP CONSISTING OF -H, -ZCO2H, -CH2CH2N(CH2CO2H)2 AND
 2. The process of claim 1 wherein the chelating agent is of the group consisting of nitrilotriacetic acid, iminodiacetic acid, nitrilotripropionic ethylenediamine tetraacetic acid, Alpha -benzylnitrilotriacetic acid, Alpha -isobutyl nitrilotriacetic acid, nitrilopropionicdiacetic acid, picolinic acid, and N-2-picolyl iminodiacetic acid.
 3. The process of claim 2 wherein the reductive coupling is carried out at a temperature in the range of about 25* to 80*C.
 4. The process of claim 3 wherein the chelating agent is nitrilotriacetic acid.
 5. The process of claim 4 wherein the oxidized coupling agent is regenerated by subsequent catalytic hydrogenation and is thereafter recycled into subsequent reductive coupling reactions. 